The present disclosure is directed to improving cut length accuracy in the cutoff knife of a corrugator dry end where the incoming output webs or “outs” may be subject to web tension change pulses that affect sheet length.
In the dry end conversion of a corrugated paperboard web, the continuously running web which has been slit along its length, is pulled into and through a rotary cutoff knife, typically having upper and lower knife levels, the web being cut crosswise into sheets of selected lengths. Such sheets are conveyed into a downstream stacker where stacks of sheets are formed and transferred away for further processing. In a typical corrugator dry end, the cutoff knife comprises a pair of counter rotating cylinders carrying helical cutting blades. A variable speed drive controls cutoff knife speed to cut sheets of widely varying lengths from the running web at both knife levels.
In such a system, the web upstream of the slit line is joined such that the output webs move together, each output web utilizing a separate driven infeed pull roll nip that imposes a first force on the output web and directs the output web into the cutoff knife. A driven outfeed or exit nip downstream of the cutoff knife engages the lead edge of the output web and imposes a second force on the output web to control the sheets after they are cut and to pull a gap between each cut sheet and the leading edge of the output web moving through the knife. The output web is thus pulled by the sum of the first and second forces until the sheet is cut. However, the output web is pulled only by the first force until the lead edge of the output web reaches the outfeed nip.